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There are, of course, rumors of alien contacts. There's Area 51, the Air Force base in Nevada, where the government supposedly stores aliens in freezers. And there was that mysterious crash landing in Roswell, New Mexico, in 1947—and mutilated cows in Colorado. In a recent poll, one in four Americans said they believed the planet has already been visited by an extraterrestrial. For the rest of us, though, aliens are relegated to the realm of fiction, and UFO sightings are simply hoaxes or events that have some unknown but natural explanation.

That doesn't mean alien life couldn't be real. Scientists are taking that possibility seriously and are looking beyond our planet for evidence of extraterrestrials. "Long ago people suspected that there might be life in other places," says Mary Voytek, NASA's head astrobiologist. "I think it's a fundamental question everyone has: Are we unique?"

What happens if the answer to that question is no? What if we finally discover we're not alone? Believe it or not, there is a plan.

The idea there might be other creatures in the universe has been around since at least the fifth century B.C., when the Greek philosopher Democritus posited "innumerable worlds of different sizes," not all of which were devoid of life. Four hundred years later, the Roman poet Titus Lucretius Carus wrote of "other worlds" with "different tribes of men, kinds of wild beasts."

In the 17th century, German astronomer Johannes Kepler, in what is regarded as the first science fiction story, wrote about a voyage to the Moon in which travelers encountered reptile-like creatures. At the end of that century, Dutch mathematician Christiaan Huygens wrote a book speculating on the conditions on other planets and concluding that some of them must harbor life.

Aiming his telescope at Mars in 1894, the American astronomer Percival Lowell saw a web of what he thought were canals—structures so elaborate they could have been built only by beings with intelligence.

With the founding of NASA and other space agencies in the 20th century, people began to explore the solar system and actively search for alien life. We sent satellites to photograph other planets and robots to explore their surfaces. Astronauts walked on the Moon and brought back rocks and dust. Scientists found evidence of water on the Moon and Mars, as well as on Jupiter's moon Europa. Amino acids were discovered in meteorites that had fallen to Earth. Ever more powerful telescopes and new ways to analyze their readings have led to the discovery of hundreds of planets orbiting other stars. Though no one has found life anywhere other than on Earth, that discovery seems just around the corner.

The search for extraterrestrials takes two broad forms. NASA and other government-funded space agencies are concentrating their search on simple, microscopic life that may have existed—or may still exist—close to home, on a planet or moon in our solar system. Other scientists search for signs of creatures a bit more like us—beings that may themselves be searching for other intelligent life-forms.

The most ambitious search began in 1960, when astronomer Frank Drake pointed a radio telescope at two stars similar to our sun and listened for some kind of "signature of intelligence." He tuned in to a frequency of 1,420 megahertz, which is the radio wave emitted by cold hydrogen gas, chosen because of hydrogen's abundance in the universe. At the time it was the best guess of the mutually intelligible signal an alien race might use to contact Earth.